The present invention relates to cellulose formate fibers and to fibers of cellulose regenerated from this formate, and also to processes for obtaining such fibers when they are used starting from liquid-crystal solutions of cellulose formate, i.e. of solutions having a high concentration of polymer.
Liquid-crystal solutions of polymers, in the field of spinning, are used in known manner for obtaining what are called "technical" fibers having very high properties of tenacity and of tensile modulus, combined with low elongations at break, as has been shown in particular by patents U.S. Pat. No. 3,767,756, which relates to aramid fibers, or U.S. Pat. No. 4,746,694, which relates to aromatic polyester fibers. The spinning of liquid-crystal solutions based on cellulose also makes it possible to obtain cellulose fibers having high mechanical properties, as described, for example, in International Patent Application PCT/CH85/00065, published under the number WO 85/05115.
This application WO 85/05115, or equivalent patents EP-B-179 822 and U.S. Pat. No. 4,839,113, disclose obtaining liquid-crystal solutions of cellulose formate by reacting cellulose with formic acid and phosphoric acid. These solutions may be spun using what is called the "dry-jet-wet spinning" technique to obtain cellulose formate fibers, as well as regenerated cellulose fibers after regeneration treatment of these formate fibers. Compared with conventional cellulose fibers such as rayon or viscose fibers, spun from non-liquid-crystal solutions, the fibers described in WO 85/05115 are characterized, owing to the liquid-crystal nature of the spinning solutions from which they have originated, by a far more ordered or oriented structure, and hence by a substantially greater strength and modulus: their tenacities may vary, for example, from 50-60 cN/tex to 80-100 cN/tex, or even more, depending on whether they are formate fibers or fibers of cellulose regenerated from this formate, their initial moduli possibly attaining 3000 to 3500 cN/tex; their values of elongation at break, in return, are low, of the order of 3% to 4%.
To obtain these high-strength, high-modulus fibers, the coagulation step is performed in acetone. Now, acetone is a relatively costly, volatile product, which furthermore involves a risk of explosion which requires special safety measures. Such disadvantages are not peculiar to acetone, but common, at least for some of them, to numerous organic liquids used in the spinning industry, in particular as coagulating agents.
It was therefore entirely desirable to find an alternative to the use of acetone by replacing this acetone with a coagulating agent which would be more advantageous from an industrial point of view and easier to use, even at the expense of a reduction in the mechanical properties described previously.
It turns out that simply replacing acetone with water in the spinning process according to the aforementioned application WO 85/05115 results in fibers having very poor mechanical properties and which arc of no real industrial interest: firstly their tenacity is clearly insufficient to meet "technical" applications (such as for reinforcing pneumatic tires), and secondly their elongation at break is too poor for "textile" applications (for example in the clothing industry), which require high values of elongation at break.